DE-PS 697,341 discloses a device based on the principle of determining the fraction of liquid fuel in a fuel tank from a piston displacement and the pressure change in the gas-filled tank space which results from this displacement. In this device, the piston is brought to a defined position by manual operation against the action of a spring, from which it is displaced by the spring force during measurement.
A similar device is disclosed in DE C2 29 53 903, in which the piston is displaced from a defined position by an air motor which operates at a pressure above or below atmospheric pressure. This device includes pressure sensors and displacement sensors, whose output signals may also be introduced, as the case may be, into an electronic control device for further processing.
Devices with manual displacement of the piston are out-of-date, and the displacement of the piston by means of an air motor takes up a large space, and can only be connected to the gas-filled tank space. Hence, such devices are not currently in use.